Using data from the North Carolina "Pregnancy, Infection, and Nutrition" study, we re-examined the question of whether PCBs or DDT in breast milk produce detectable changes in growth or cognitive development. In previous work with older methods for chemical analysis, we saw no such effects, but this study used more modern, sensitive methods. We saw no associations between infant growth or cognitive development and exposure to PCBs or DDT in breast milk in these new data. Thus, the previous negative findings do not appear to be due to insensitive methods fro chemical analysis.